We continued our examination of the decrease in binding of the cocaine derivative [3H]WIN 35,428 to the dopamine (DA) transporter in homogenized striatum and nucleus accumbens, 1, 10, 30, or 60 days after a series of IV infusions of cocaine. The binding of [3H)WIN 35,428 was reduced by 30% in the nucleus accumbens 10, 30, and 60 days after of cocaine, but not 1 day later or after saline. No change occurred in the striatum at any time. The persistent neurochemical decrease after withdrawal of cocaine may suggest that the DA transporters in the two regions are regulated differently. Two related studies are in progress. The first is an autoradiographic study of [3H)WIN 35,428 binding in brains of rats injected intravenously with the isotope 1 or 10 days after the last IV infusion of cocaine. The second is the determination of the distribution in the brain of the mRNA for the dopamine transporter using in situ hybridization in the brains of rats 10 days after the repeated infusions of cocaine or saline. The peptide neurotensin (NT) resides in part in the mesocorticolimbic DA neurons. The relative abundances of NT and DA within these neurons coupled with the abilities of NT to activate this region suggested that a reciprocal modulation between NT and DA might occur. Multiple IV infusions of cocaine decreased NT binding at their cell bodies but increased it markedly at their terminal fields. Ten days after the withdrawal of cocaine, recovery of NT binding was apparent at the cell body region, but binding at the terminal fields was enhanced further, suggesting that withdrawal from cocaine might decrease the synthesis and/or release of NT. Our preliminary findings show that NT accumulates in the prefrontal cortex and in the substantia nigra, the terminal fields of these neurons, 10 days after cocaine is withdrawn. It seems that these neurons synthesize NT, after withdrawal of cocaine, but may not be able to release it. We are also analyzing NT acccumulation and binding to 30 and 60 days after cocaine to determine when recovery occurs.